A solar cell is expected as a new energy source because it can directly convert light from the sun, which is clean and inexhaustible energy source, into electricity.
Output per solar cell is as small as several W. Accordingly, when used for a power source of houses or buildings, such solar cells are usually used as a solar cell module in which the output is increased to several hundreds W by electrically connecting the plurality of solar cells in series or in parallel. FIG. 1 is a view showing a part of a conventional solar cell module. FIG. 2 is a cross-sectional view taken along line X-X′ of FIG. 1. The plurality of solar cells 101 are electrically connected with each other in a manner that collecting electrodes (finger electrodes 111 or bus bar electrodes 121) formed respectively on the surfaces of the solar cells 101 are connected to each other with wiring members 141. The collecting electrodes are print-formed in a width roughly equal to or wider than the width of the wiring members 141.
Here, each of the wiring members 141 is an electric conductor in which the periphery of a low resistive body 141a made of copper or the like is coated with a solder 141b of tin, silver, copper or the like, as shown in FIG. 2. In addition, the solar cell 101 is sealed by a translucent filling material, such as EVA, which is filled between a translucent surface member such as glass or transparent plastic, and a back surface member made of a resin film such as polyethylene terephthalate film, a steel plate or a glass plate. Here, linear expansion coefficients of the wiring member 141 being a copper foil, for example, and the solar cell 101 formed of a crystal silicon substrate are 17.8 ppm/° C. and 4.2 ppm/° C., respectively, and differ by a factor of more than four. Accordingly, the degrees of expansion and shrinkage due to heating and cooling at a time of soldering to connect the wiring member 141 to the bus bar electrode 121 formed on the solar cell 101 differ among these materials. As a result, warping stress is generated in the solar cell 101, so that a cell crack or an electrode peeling occurs. Particularly, as a thickness of the solar cell becomes thinner in order to reduce the production cost of the solar cell, this problem becomes more significant, and thus there has been a problem of decreasing production yield by a solar cell crack and the like.
Moreover, when the solar cell module is designed to increase output thereof by increasing thickness of the wiring member for reducing serial resistance of the wiring member, a problem of easily generating warpage of a solar cell has also existed in a similar way.
In addition, a lead-tin eutectic solder having a eutectic point of a melting point of 183° C., and having high reliability and good workability has been used conventionally as adhesive material of the wiring member in modularization of solar cells; however, in recent years, replacement to solder materials without containing lead has been promoted in order to protect the environment. Nowadays, a tin-silver-copper eutectic solder having a eutectic point of a melting point of 217° C. is used in many cases. In solder bonding operation using the tin-silver-copper eutectic solder, heating to approximately 240° C. is generally performed. Thus, an operation temperature which is approximately 30° C. or more higher than that of a conventional lead-tin eutectic solder has made it more difficult to solve the warpage problem of the above-described solar cell.
Meanwhile, a solar cell device aiming to resolve the solar cell crack problem caused by thickening the copper foil used for a conventional wiring member has been suggested (for example, see Patent Document 1).
This suggested solar cell device is a solar cell device in which a plurality of solar cells are connected to each other by wiring members in a manner that: the wiring members having approximately the same length are soldered in advance to each of the solar cells; and thereafter, the wiring members connected to a light-receiving surface side of a solar cell and to a back surface side of another solar cell are connected by another wiring member.
According to this method, the separate wiring members are connected to the electrodes of the light-receiving surface sides and the back surface sides, and then these wiring members are connected to each other. Accordingly, compression stress applied to one solar cell by thermal expansion and shrinkage of the wiring members is limited to compression stress caused in only the one solar cell. As a result, the solar cell crack is resolved because the adjacent solar cells are not pulled towards each other.
Patent Document 1: Japanese Patent Application Publication No. 2002-3593139